Slurry phase polymerisation process

ABSTRACT

Polymerization process in a loop reactor of an olefin monomer optionally together with an olefin commoner in the presence of a polymerization catalyst in a diluent to produce a slurry containing solid particulate olefin polymer and the diluent. The Froude number is maintained in the range of 2-15.

This application is a continuation of application Ser. No. 14/560,368filed Dec. 4, 2014, which is a continuation of application Ser. No.14/047,718 filed Oct. 7, 2013 (U.S. Pat. No. 8,927,665), which is acontinuation of application Ser. No. 12/801,885 filed Jun. 30, 2010(U.S. Pat. No. 8,580,202, which is a continuation of Ser. No. 12/382,656filed Mar. 20, 2009 (U.S. Pat. No. 7,790,119), which is a continuationof Ser. No. 11/667,829 filed Dec. 12, 2007 (U.S. Pat. No. 7,632,899),which is a 371 of PCT/GB2005/004484 filed Nov. 22, 2005, which claimspriority to British Patent Application No. 0426058.4 filed Nov. 26,2004, the entire contents of each of which are hereby incorporated byreference.

The present invention is concerned with olefin polymerisation in slurryphase loop reactors.

Slurry phase polymerisation of olefins is well known wherein an olefinmonomer and optionally olefin comonomer are polymerised in the presenceof a catalyst in a diluent in which the solid polymer product issuspended and transported.

This invention is specifically related to polymerisation in a loopreactor where the slurry is circulated in the reactor typically by meansof a pump or agitator. Liquid full loop reactors are particularly wellknown in the art and are described for example in U.S. Pat. Nos.3,152,872, 3,242,150 and 4,613,484.

Polymerisation is typically carried out at temperatures in the range50-125° C. and at pressures in the range 1-100 bara. The catalyst usedcan be any catalyst typically used for olefin polymerisation such aschromium oxide, Ziegler-Natta or metallocene-type catalysts. The productslurry comprising polymer and diluent, and in most cases catalyst,olefin monomer and comonomer can be discharged intermittently orcontinuously, optionally using concentrating devices such ashydrocyclones or settling legs to minimise the quantity of fluidswithdrawn with the polymer.

The loop reactor is of a continuous tubular construction comprising atleast two, for example four, vertical sections and at least two, forexample four, horizontal sections. The heat of polymerisation istypically removed using indirect exchange with a cooling medium,preferably water, in jackets surrounding at least part of the tubularloop reactor. The volume of the loop reactor can vary but is typicallyin the range 20 to 120 m³; the loop reactors of the present inventionare of this generic type.

Maximum commercial scale plant capacities have increased steadily overthe years. Growing operating experience over the last few decades hasled to operation of increasingly high slurry and monomer concentrationsin reaction loops. The increase in slurry concentrations has typicallybeen achieved with increased circulation velocities achieved for exampleby higher reactor circulation pump head or multiple circulation pumps asillustrated by EP 432555 and EP 891990. The increase in solids loadingis desirable to increase reactor residence time for a fixed reactorvolume and also to reduce downstream diluent treatment and recyclingrequirements. The increased velocity and head requirement of the loophas however led to increasing pump design sizes and complexity, andenergy consumptions as slurry concentrations increase. This has bothcapital and operating cost implications.

Historically the circulation velocity in the reaction loop has typicallybeen maximised to ensure maintenance of good thermal, compositional andparticle distribution across the reactor cross-section, particularly theavoidance of solids settling, stable flow characteristics, or excessivesolids concentrations at the pipe wall rather than reduced to minimisepressure drop/power in the polymerisation loop.

Inadequate cross-sectional distribution could lead to increased fouling,reduced heat transfer and reduced polymer productivity and homogeneity.Construction and commissioning of new commercial plants is veryexpensive and therefore new designs seek to avoid or minimise changes tooperating parameters that are seen to increase risk to the successfuloperation of the new unit.

In accordance with the present invention there is provided a processcomprising polymerising in a loop reactor an olefin monomer optionallytogether with an olefin comonomer in the presence of a polymerisationcatalyst in a diluent to produce a slurry comprising solid particulateolefin polymer and the diluent wherein the Froude number is maintainedat or below 20.

One advantage of the present invention is that the specific energyconsumption of the reactor (i.e. the energy consumed per unit weight ofpolymer produced) is reduced whilst maintaining a given reactorresidence time and avoiding unacceptable reactor fouling. The inventionis especially advantageous when it is desired to design and operate aplant at high solids loadings when it has previously been considerednecessary to use what have now been found to be excessively high loopcirculation velocities.

This invention relates to a method and apparatus for continuouspolymerization of olefins, preferably alpha mono olefins, in anelongated tubular closed loop reaction zone. The olefin(s) iscontinuously added to, and contacted with, a catalyst in a hydrocarbondiluent. The monomer(s) polymerise to form a slurry of solid particulatepolymer suspended in the polymerisation medium or diluent.

Typically, in the slurry polymerisation process of polyethylene, theslurry in the reactor will comprise the particulate polymer, thehydrocarbon diluent(s), (co) monomer(s), catalyst, chain terminatorssuch as hydrogen and other reactor additives In particular the slurrywill comprise 20-75, preferably 30-70 weight percent based on the totalweight of the slurry of particulate polymer and 80-25, preferably 70-30weight percent based on the total weight of the slurry of suspendingmedium, where the suspending medium is the sum of all the fluidcomponents in the reactor and will comprise the diluent, olefin monomerand any additives; the diluent can be an inert diluent or it can be areactive diluent in particular a liquid olefin monomer; where theprincipal diluent is an inert diluent the olefin monomer will typicallycomprise 2-20, preferably 4-10 weight percent of the total weight of theslurry.

The slurry is pumped around the relatively smooth path-endless loopreaction system at fluid velocities sufficient to (i) maintain thepolymer in suspension in the slurry and (ii) to maintain acceptablecross-sectional concentration and solids loading gradients.

It has, now been found that cross-sectional slurry concentrationdistributions (as evidenced by fouling, flow variations and/or heattransfer) can be maintained within acceptable operating limits whilstmaintaining the Froude number in the reactor loop below 20, preferablybetween 2 and 15, most preferably between 3 and 10. This is contrary towhat the man skilled in the art would believe to be the ease in thelight of conventional process conditions where the Froude number istypically above 20, for example above 30, typically in the range 30-40.

The Froude number will be maintained at or below 20, for example in therange 20 to 1 preferably in the range 15 to 2, more preferably in therange 10 to 3. The Froude number is a dimensionless parameter indicativeof the balance between the suspension and settling tendencies ofparticles in a slurry. It provides a relative measure of the momentumtransfer process to the pipe wall from particles compared to the fluid.Lower values of the Froude number indicate stronger particle-wall(relative to fluid-wall) interactions. The Froude number (Fr) is definedas v²/(g(s−1)D) where v is the average velocity of the slurry, g is thegravitational constant, s is the specific gravity of the solid in thediluent and D is the internal pipe diameter. The specific gravity of thesolid polymer which is the ratio of the density of the polymer to thedensity of water is based on the annealed density of the degassedpolymer after being substantially devolatilised and immediately prior toany extrusion as measured using method ISO1183A.

The solids concentration in the slurry in the reactor will typically beabove 20 vol %, preferably about 30 volume %, for example 20-40 volume%, preferably 25-35 volume % where volume % is [(total volume of theslurry−volume of the suspending medium)/(total volume of theslurry)]×100. The solids concentration measured as weight percentagewhich is equivalent to that measured as volume percentage will varyaccording to the polymer produced but more particularly according to thediluent used. Where the polymer produced is polyethylene and the diluentis an alkane, for example isobutane it is preferred that the solidsconcentration is above 40 weight % for example in the range 40-60,preferably 45%-55 weight % based on the total weight of the slurry.

It is a particular feature of the present invention that operation ofthe slurry phase polymerisation at low Froude numbers enables thereactor to be run at high solids loading. A preferred embodiment of thepresent invention is a process comprising polymerising in a loop reactoran olefin monomer, in particular ethylene, optionally together with anolefin comonomer in the presence of a polymerisation catalyst in adiluent, particularly isobutane, to produce a slurry comprising solidparticulate olefin polymer and the diluent wherein the Froude number ismaintained at or below 20, particularly in the range 3 to 10 and thesolids concentration in the reactor is in the range 25-35% volume.

It is a further feature of the present invention that operation of theprocess can be carried out in larger diameter reactors than areconventionally used in slurry polymerisation without any significantproblems particularly from fouling at the reactor walls. For example,reactors having internal diameters over 500 millimeters, in particularover 600 for example between 600 and 750 millimeters can be used wherehistorically there would have been increased concern. A furtheradvantage of this invention is therefore that high slurry concentrationsat relatively low circulation velocities and/or relatively high reactorloop diameters can be achieved. A further embodiment of the presentinvention is a process comprising polymerising in a loop reactor anolefin monomer optionally together with an olefin comonomer in thepresence of a polymerisation catalyst in a diluent to produce a slurrycomprising solid particulate olefin polymer and the diluent wherein theFroude number is maintained at or below 20, preferably 3-10 and theinternal diameter of the reactor is in the range 600-750 millimeters.

It has been found that reactors can be designed and operated at specificpressure drop both per unit reactor length and per mass of polymer andtotal pressure drop for the loop less than that taught as beingrequired, particularly at high solids loadings and/or large reactordiameters. This invention permits total loop pressure drops of less than1.3 bar, particularly less than 1 bar even for polymer production ratesof above 25, even above 45 tonnes per hour It is possible to employ oneor more than one pump in the loop preferably on one or more horizontalsections; these can be located on the same horizontal section or ondifferent sections. The pump or pumps can be of the same diameter orlarger or smaller diameter preferably of the same diameter as theinternal diameter of the section of the reactor where the pump or pumpsare located. It is preferable to employ a single pump and it is afeature of the present invention that requirements for number and powerof pump(s) is less onerous than for conventional processes.

Reactor size is typically over 20 m³ in particular over 50 m³ forexample 75-150 m³ preferably in the range 100-125 m³

The discovery of an operating window at low Froude numbers enablesacceptable design bases for larger reactor diameters to be defined. Thisenables reactor volumes, for example of greater than 80 m³ to be builtwith a reactor length to internal diameter ratio of less than 500,preferably less than 400 more preferably less than 250. Reduction inreactor length to internal diameter ratio minimises compositionalgradients around the reaction loop and enables production rates ofgreater than 25 tonnes (per reactor) per hour to be achieved with only asingle point of introduction for each reagent around the reaction loop.Alternatively it is possible to have multiple inlets into the loopreactor for reactants (e.g. olefins), catalyst, or other additives.

The pressure employed in the loop will be sufficient to maintain thereaction system ‘liquid full’ i.e. there is substantially no gas phase.Typical pressures used are between 1-100 bara, preferably between 30 to50 bara. In ethylene polymerization the ethylene partial pressure willtypically be in the range 0.1 to 5 MPa, preferably from 0.2 to 2 MPa,more particularly from 0.4 to 1.5 MPa. The temperatures selected aresuch that substantially all of the polymer produced is essentially (i)in a non-tacky and non-agglomerative solid particular form and (ii)insoluble in the diluent. The polymerization temperature depends on thehydrocarbon diluent chosen and the polymer being produced. In ethylenepolymerisation the temperature is generally below 130 C, typicallybetween 50 and 125 C, preferably between 75 and 115 C. For example inethylene polymerisation in isobutane diluent, the pressure employed inthe loop is preferably in the range 30-50 bara, the ethylene partialpressure is preferably in the range 0.2-2 MPa and the polymerisationtemperature is in the range 75-115 C. The space time yield which isproduction rate of polymer per unit of loop reactor volume for theprocess of the present invention is in the range 0.1-0.4 preferably0.2-0.35 ton/hour/m³.

The process according to the invention applies to the preparation ofcompositions containing olefin (preferably ethylene) polymers which cancomprise one or a number of olefin homo-polymers and/or one or a numberof copolymers. It is particularly suited to the manufacture of ethylenepolymers and propylene polymers. Ethylene copolymers typically comprisean alpha-olefin in a variable amount which can reach 12% by weight,preferably from 0.5 to 6% by weight, for example approximately 1% byweight.

The alpha mono-olefin monomers generally employed in such reactions areone or more 1-olefins having up to 8 carbon atoms per molecule and nobranching nearer the double bond than the 4-position. Typical examplesinclude ethylene, propylene, butene-1, pentene-1, hexene-1 and octene-1,and mixtures such as ethylene and butene-1 or ethylene and hexene-1.Butene-1, pentene-1 and hexene-1 are particularly preferred comonomersfor ethylene copolymerisation.

Typical diluents employed in such reactions include hydrocarbons having2 to 12, preferably 3 to 8, carbon atoms per molecule, for examplelinear alkanes such as propane, n-butane, n-hexane and n-heptane, orbranched alkanes such as isobutane, isopentane, toluene, isooctane and2,2,-dimethylpropane, or cycloalkanes such as cyclopentane andcyclohexane or their mixtures. In the case of ethylene polymerization,the diluent is generally inert with respect to the catalyst, cocatalystand polymer produced (such as liquid aliphatic, cycloaliphatic andaromatic hydrocarbons), at a temperature such that at least 50%(preferably at least 70%) of the polymer formed is insoluble therein.Isobutane is particularly preferred as the diluent for ethylenepolymerisation.

The operating conditions can also be such that the monomers (e.g.ethylene, propylene) act as the diluent as is the case in so called bulkpolymerisation processes. The slurry concentration limits in volumepercent have been found to be able to be applied independently ofmolecular weight of the diluent and whether the diluent is inert orreactive, liquid or supercritical. Propylene monomer is particularlypreferred as the diluent for propylene polymerisation

Methods of molecular weight regulation are known in the art. When usingZiegler-Natta, metallocene and tridentate late transition metal typecatalysts, hydrogen is preferably used, a higher hydrogen pressureresulting in a lower average molecular weight. When using chromium typecatalysts, polymerization temperature is preferably used to regulatemolecular weight.

In commercial plants, the particulate polymer is separated from thediluent in a manner such that the diluent is not exposed tocontamination so as to permit recycle of the diluent to thepolymerization zone with minimal if any purification. Separating theparticulate polymer produced by the process of the present inventionfrom the diluent typically can be by any method known in the art forexample it can involve either (i) the use of discontinuous verticalsettling legs such that the flow of slurry across the opening thereofprovides a zone where the polymer particles can settle to some extentfrom the diluent or (ii) continuous product withdrawal via a single ormultiple withdrawal ports, the location of which can be anywhere on theloop reactor but is preferably adjacent to the downstream end of ahorizontal section of the loop. Any continuous withdrawal ports willtypically have an internal diameter in the range 2-25, preferably 4-15,especially 5-10 cm. This invention permits large scale polymerisationreactors to be operated with low diluent recover requirements. Theoperation of large diameter reactors with high solids concentrations inthe slurry minimises the quantity of the principal diluent withdrawnfrom the polymerisation loop. Use of concentrating devices on thewithdrawn polymer slurry, preferably hydrocylones (single or in the caseof multiple hydrocyclones in parallel or series), further enhances therecovery of diluent in an energy efficient manner since significantpressure reduction and vaporisation of recovered diluent is avoided.

It has been found that both the slurry concentration and the Froudenumber in the reactor loop can be optimised by controlling the averageparticle size and/or the particle size distribution of the powder withinthe reactor loop. The principal determinant of the average particle sizeof the powder is the residence time in the reactor. The particle sizedistribution of the catalyst can be affected by many factors includingthe particle size distribution of the catalyst fed to the reactor, theinitial and average catalyst activity, the robustness of the catalystsupport and susceptibility of the powder to fragment under reactionconditions. Solids separating devices (such as hydrocyclones) can beused on the slurry withdrawn from the reactor loop to further assist incontrol of the average particle size and the particle size distributionof the powder in the reactor. The location of the withdrawal point forthe concentrating device and the design and operating conditions of theconcentrating device system, preferably the at least one hydrocyclonerecycle loop, also enables the particle size and particle sizedistribution within the reactor to be controlled. The average particlesize is preferably between 100 and 1500 microns, most preferably between250 and 1000 microns.

The withdrawn, and preferably concentrated, polymer slurry isdepressurised, and optionally heated, prior to introduction into aprimary flash vessel. The stream is preferably heated afterdepressurisation.

The diluent and any monomer vapors recovered in the primary flash vesselare typically condensed, preferably without recompression and reused inthe polymerization process. The pressure of the primary flash vessel ispreferably controlled to enable condensation with a readily availablecooling medium (e.g. cooling water) of essentially all of the flashvapour prior to any recompression, typically such pressure in saidprimary flash vessel will be 4-25, for example 10-20, preferably 15-17bara. The solids recovered from the primary flash vessel is preferablypassed to a secondary flash vessel to remove residual volatiles.Alternatively the slurry may be passed to a flash vessel of lowerpressure than in the above mentioned primary vessel such thatrecompression needed to condense the recovered diluent. Use of a highpressure flash vessel is preferred.

The process according to the invention can be used to produce resinswhich exhibit specific density in the range 0.890 to 0.930 (lowdensity), 0.930 to 0.940 (medium density) or 0.940 to 0.970 (highdensity).

The process according to the invention is relevant to all olefinpolymerisation catalyst systems, particularly those chosen from theZiegler-type catalysts, in particular those derived from titanium,zirconium or vanadium and from thermally activated silica or inorganicsupported chromium oxide catalysts and from metallocene-type catalysts,metallocene being a cyclopentadienyl derivative of a transition metal,in particular of titanium or zirconium.

Non-limiting examples of Ziegler-type catalysts are the compoundscomprising a transition metal chosen from groups IIIB, IVB, VB or VIB ofthe periodic table, magnesium and a halogen obtained by mixing amagnesium compound with a compound of the transition metal and ahalogenated compound. The halogen can optionally form an integral partof the magnesium compound or of the transition metal compound.

Metallocene-type catalysts may be metallocenes activated by either analumoxane or by an ionising agent as described, for example, in PatentApplication EP-500,944-A1 (Mitsui Toatsu Chemicals).

Ziegler-type catalysts are most preferred. Among these, particularexamples include at least one transition metal chosen from groups IIIB,IVB, VB and VIB, magnesium and at least one halogen. Good results areobtained with those comprising:

from 10 to 30% by weight of transition metal, preferably from 15 to 20%by weight,

from 20 to 60% by weight of halogen, the values from 30 to 50% by weightbeing preferred,

from 0.5 to 20% by weight of magnesium, usually from 1 to 10% by weight,

from 0.1 to 10% by weight of aluminium, generally from 0.5 to 5% byweight,

the balance generally consists of elements arising from the productsused for their manufacture, such as carbon, hydrogen and oxygen. Thetransition metal and the halogen are preferably titanium and chlorine.

Polymerisations, particularly Ziegler catalysed ones, are typicallycarried out in the presence of a cocatalyst. It is possible to use anycocatalyst known in the art, especially compounds comprising at leastone aluminium-carbon chemical bond, such as optionally halogenatedorganoaluminium compounds, which can comprise oxygen or an element fromgroup I of the periodic table, and aluminoxanes. Particular exampleswould be organoaluminium compounds, of trialkylaluminiums such astriethylaluminium, trialkenylaluminiums such as triisopropenylaluminium,aluminium mono- and dialkoxides such as diethylaluminium ethoxide, mono-and dihalogenated alkylaluminiums such as diethylaluminium chloride,alkylaluminium mono- and dihydrides such as dibutylaluminium hydride andorganoaluminium compounds comprising lithium such as LiAl(C₂ H₅)₄.Organoaluminium compounds, especially those which are not halogenated,are well suited. Triethylaluminium and triisobutylaluminium areespecially advantageous.

The chromium-based catalyst is preferred to comprise a supportedchromium oxide catalyst having a titania-containing support, for examplea composite silica and titania support. A particularly preferredchromium-based catalyst may comprise from 0.5 to 5 wt % chromium,preferably around 1 wt % chromium, such as 0.9 wt % chromium based onthe weight of the chromium-containing catalyst. The support comprises atleast 2 wt % titanium, preferably around 2 to 3 wt % titanium, morepreferably around 2.3 wt % titanium based on the weight of the chromiumcontaining catalyst. The chromium-based catalyst may have a specificsurface area of from 200 to 700 m.sup.2/g, preferably from 400 to 550m.sup.2/g and a volume porosity of greater than 2 cc/g preferably from 2to 3 cc/g.

Silica supported chromium catalysts are typically subjected to aninitial activation step in air at an elevated activation temperature.The activation temperature preferably ranges from 500 to 850.degree. C.,more preferably 600 to 750.degree. C.

The reactor loop can be used to make monomodal or multimodal, forexample bimodal, polymers. The multi-modal polymers can be made in asingle reactor or in multiple reactors. The reactor system can compriseone or more loop reactors connected in series or in parallel. Thereactor loop may also be preceded or followed by a polymerisationreactor that is not a loop reactor.

In the case of series reactors, a first reactor of the series issupplied with catalyst and the cocatalyst in addition to the diluent andmonomer, and each subsequent reactor is supplied with, at least,monomer, in particular ethylene and with the slurry arising from apreceding reactor of the series, this mixture comprising the catalyst,the cocatalyst and a mixture of the polymers produced in a precedingreactor of the series. It is optionally possible to supply a secondreactor and/or, if appropriate, at least one of the following reactorswith fresh catalyst and/or cocatalyst. However, it is preferable tointroduce the catalyst and the cocatalyst exclusively into a firstreactor.

In the case where the plant comprises more than two reactors in series,the polymer of highest melt index and the polymer of lowest melt indexcan be produced in two adjacent or non-adjacent reactors in the series.Hydrogen is maintained at (i) a low (or zero) concentration in thereactor(s) manufacturing the high molecular weight components, e.g.hydrogen percentages including between 0-0.1 vol % and at (ii) a veryhigh concentration in the reactor(s) manufacturing the low molecularweight components e.g. hydrogen percentages between 0.5-2.4 vol %. Thereactors can equally be operated to produce essentially the same polymermelt index in successive reactors.

Particular sensitivity to operating at reduced Froude numbers (andassociated cross-sectional compositional, thermal or particulategradients) has however been related to production of polymer resinswhere polymer of either high or low molecular weight resins has beenknown to lead to increased fouling concerns. Particularly when producingpolymers of molecular weights less than 50 kDaltons or greater than 150kDaltons. These concerns have particularly been confirmed to beaccentuated, at low polymer solids concentrations in the reactor loop.When producing polymers of molecular weights less than 50 kDaltons orgreater than 200 kDa (or melt index below 0.1 and above 50) in largediameter reactors it has however surprisingly been discovered thatfouling is decreased when solids loadings are increased to above 20 vol%, particularly above 30 vol %.

The invention will now be illustrated by reference to the followingexample.

EXAMPLE 1

In an elongated closed loop tubular reactor having an internal diameterof 711 millimeters and a volumetric capacity of 62 m³, ethylene wascopolymerised with hexene-1 at a temperature of 85° C. and a pressure of30 bara in isobutane as diluent and using a Ziegler-Natta catalyst toproduce a copolymer The Froude Number was maintained below 10 for aperiod of six days, with a essentially constant solids loading, of about44.5 wt %. The reactor circulation pump power as measured by the amptransducer on the pump motor control system (see Table 1) and readingsof voltage at the motor control system and heat transfer coefficient asmeasured by monitoring coolant water flow and coolant water temperaturechange compared to reactor temperature remained stable to within +/−0.6%and +/−0.6% respectively, indicating that there was no detectablefouling of the reactor as evidenced, by a build up of polymer on thewalls of the reactor, and that flow was stable and well distributed asevidenced by the stable pump power readings.

TABLE 1 Date Day 1 21:00:00 Day 6 09:00:00 5.5 days amps 32.18 32 −0.6%solids, wt % 44.5 44.5 0

This provides evidence of heat transfer coefficient stability and pumppower stability at low Froude numbers.

The invention claimed is:
 1. A process comprising polymerizing in a loopreactor an olefin monomer optionally together with an olefin comonomerin the presence of a polymerization catalyst in a diluent to produce aslurry comprising solid particulate olefin polymer and the diluentwherein the Froude number is in the range 2 to 15, wherein the loopreactor has an internal diameter of over 500 millimeters, wherein thesolids concentration of the slurry in the loop reactor is above 20 vol %and the average particle size is between 100 and 1500 microns, andwherein the loop reactor is one of two or more reactors connected inseries and being used to produce a multimodal polymer, and at least oneof the following applies: i) hydrogen is maintained at a low (or zero)concentration of 0-0.1 vol % in the reactor(s) manufacturing the highmolecular weight components and hydrogen is maintained at a highconcentration of 0.5-2.4 vol % in the reactor(s) manufacturing the lowmolecular weight components, ii) there is being produced a polymer withmolecular weight less than 50 kDaltons, iii) there is being produced apolymer with molecular weight greater than 200 kDaltons, iv) there isbeing produced a polymer with a melt index below 0.1, v) there is beingproduced a polymer with a melt index above
 50. 2. A process as claimedin claim 1 wherein the Froude number is in the range 3 to
 10. 3. Aprocess as claimed in claim 1 wherein the solids concentration of theslurry in the loop reactor is above 30 vol %.
 4. A process as claimed inclaim 1 wherein the solids concentration of the slurry in the loopreactor is in the range 40-60 wt % based on the total weight of theslurry.
 5. A process as claimed in claim 1 wherein the internal diameteris in the range 600 to 750 millimeters.
 6. A process as claimed in claim1 wherein the total pressure drop in the loop reactor is less than 1.3bar.
 7. A process as claimed in claim 1 wherein the reactor size isgreater than 50 m³.
 8. A process as claimed in claim 7 wherein thereactor size is in the range 75-150 m³.
 9. A process as claimed in claim1 wherein polymer slurry is withdrawn, optionally concentrated,depressurised, and optionally heated, prior to introduction into aprimary flash vessel at a pressure of 4-25 bara.
 10. A process asclaimed in claim 9 wherein the primary flash vessel is at a pressure of10-20 bara.
 11. A process as claimed in claim 9 wherein the polymerslurry stream is heated after depressurisation.
 12. A process as claimedin claim 1 wherein the space time yield is in the range 0.2-0.35ton/hour/m³.
 13. A process as claimed in claim 1 wherein the olefin isethylene.
 14. A process as claimed in claim 13 wherein the diluent isisobutane.
 15. A process as claimed in claim 1 wherein the principaldiluent is an inert diluent and the olefin monomer comprises 2-20 weightpercent of the total weight of the slurry.
 16. A process as claimed inclaim 15 wherein the olefin monomer comprises 4-10 weight percent of thetotal weight of the slurry.
 17. A process as claimed in claim 1 whereinthe catalyst is chosen from Ziegler catalysts, inorganic supportedchromium oxide catalysts and metallocene catalysts.
 18. A process asclaimed in claim 1 wherein a concentrating device is used on thewithdrawn polymer slurry.
 19. A process as claimed in claim 18 whereinthe concentrating device is a hydrocyclone.
 20. A process as claimed inclaim 1 wherein the average particle size of the polymer within the loopreactor is between 250 and 1000 microns.
 21. A process as claimed inclaim 1 wherein the particle size distribution of the polymer powder inthe loop reactor is controlled.
 22. A process as claimed in claim 21wherein the average particle size of the polymer within the loop reactoris controlled by one or more of: a) controlling the residence time inthe reactor, b) the use of concentrating devices on the slurry withdrawnfrom the reactor.
 23. A process as claimed in claim 1 wherein theproduct is withdrawn from the loop reactor continuously via a single ormultiple withdrawal ports.
 24. A process as claimed in claim 23 whereinthe withdrawal ports are adjacent to the downstream end of a horizontalsection of the loop.
 25. A process as claimed in claim 23 wherein thecontinuous withdrawal ports have an internal diameter in the range 2-25cm.
 26. A process as claimed in claim 1 wherein the loop reactor is oneof two or more loop reactors connected in series.